发布时间 : 星期六 文章PECVD镀膜更新完毕开始阅读6be815ef998fcc22bcd10dca
浙江大学硕士学位论文 王晓泉 2003年5月
太阳电池用氮化硅薄膜及氢钝化研究
摘 要
利用太阳能电池发电是解决能源问题和环境问题的重要途径之一。目前，80%以上的太阳电池是由晶体硅材料制备而成的，制备高效率低成本的晶体硅太阳能电池对于大规模利用太阳能发电有着十分重要的意义。减反射膜的制备和氢钝化是制备高效率的晶体硅太阳电池的非常重要工序之一。
本文在系统综述当前太阳电池用氮化硅薄膜研究进展、前景和面临的问题的基础上，应用PECVD（等离子体增强化学气相沉积）系统，以硅烷和氨气为气源制备了同时具有钝化作用和减反射作用的氮化硅薄膜；摸索了氮化硅薄膜相对最佳生长参数；研究了PECVD生长的氮化硅薄膜的基本物理化学性质以及在沉积的过程中，衬底温度、硅烷氨气流量比和射频功率对薄膜折射率和生长速率的影响；分别探明了氢等离子体和富氢氮化硅薄膜对太阳电池材料和器件性能的钝化作用，并就沉积薄膜后退火对电池材料和器件的影响做了初步的摸索，得到了一系列的实验结果，为开发我国自主知识产权的太阳电池工艺提供了有益的参考和指导。
本实验利用PECVD设备，制备了氮化硅薄膜，结果证实沉积的氮化硅薄膜减反射性质良好，透射率高，折射率2.1左右；薄膜表面相当平整，粗糙度大约3nm；薄膜属于非晶态，比较难晶化；薄膜的硅氮比在1.1:1至1.4:1左右，薄膜富硅；实验还研究了氮化硅薄膜的高温热稳定性，指出原始氮化硅薄膜中含有大量的氢，但是在高温处理后这些氢会从薄膜中逸失；同时，1000℃热处理可能使氮化硅薄膜发生龟裂。
实验表明，氮化硅薄膜的沉积速率随硅烷/氨气流量比增大而增大，随温度升高而略有降低,随沉积功率增大而明显增加；在衬底温度300℃，射频功率20W和硅烷氨气流量比为1：3的条件下氮化硅薄膜的沉积速率大约为8.6纳米/分。氮化硅薄膜的折射率随硅烷/氨气流量比增大而增大，随温度升高而略有增加,随沉积功率增大而略为降低。
通过测试氢等离子体钝化和氮化硅薄膜钝化的效果，实验还发现氢等离子体处理对多晶硅材料的少子寿命提高作用比较明显，但是这种提高作用与处理温度以
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浙江大学硕士学位论文 王晓泉 2003年5月
及时间的关系不大；氮化硅薄膜中的氢对单晶硅的载流子迁移率提高有一定作用，但经过高温处理后这种作用消失；氮化硅薄膜能提高单晶硅和多晶硅的少子寿命，具有表面钝化和体钝化的双重作用；氢等离子体和氮化硅薄膜都能有效地提高单晶和多晶电池的短路电流密度，进而使电池效率有不同程度(绝对转换效率0.5%~2.9%)的提高；先沉积氮化硅薄膜再氢等离子体处理能得到更好的钝化效果。
经过薄膜后退火处理发现，氮化硅薄膜经热处理后厚度降低，折射率升高,但温度达到1000℃时折射率急剧降低；沉积氮化硅薄膜后400℃退火可以促进氢扩散，提高钝化效果；超过400℃后氢开始逸失，晶体硅材料中的少子寿命急剧下降；RTP（快速热处理）处理所导致氢的逸失比常规退火处理显著。 【关键词】PECVD； 氮化硅薄膜； 氢钝化； 太阳电池
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浙江大学硕士学位论文 王晓泉 2003年5月
Research on Silicon Nitride Thin Film for Solar Cells
and Hydrogen Passivation
Abstract
Generating electricity by solar cells becomes one of the significant methods to solve the shortage of energy and environment pollution. Currently, more than 80% solar cells are made from crystalline silicon, so it is very important to develop high efficiency and low cost crystalline silicon solar cells. Preparing anti-reflective coating and hydrogen passivation are two key procedures in the process of high efficiency crystalline silicon solar cells.
The research progress, future and unsolved problems of silicon nitride thin film for solar cells were systematically reviewed in this paper. By the PECVD (Plasma Enhanced Chemical Vapor Deposition) system and the reactants of silane and ammonia, silicon nitride thin film with excellent anti-reflective and passivation effects was prepared. The relatively optimum parameters for depositing SiNx thin film and the basic physical and chemical properties of SiNx were investigated. The effects of substrate temperature, the flow ratio of silane over ammonia and the RF power on the refractivity and deposition rate were researched. The passivation results of hydrogen plasma and SiNx thin film on the solar cell materials and devices were studied, respectively. The effects of post deposition annealing on solar cell materials and devices also were discussed primarily. These experimental results offered helpful reference and guide for developing the domestic solar cell process.
The silicon nitride thin film deposited in this paper has excellent anti-reflective results and transmissivity. Its refractive index is about 2.1. The surface of thin film is very smooth, with a roughness of about 3 nm. The film is amorphous and hard to be crystallized. The atomic ratio of Si and N ranges from 1.1:1 to 1.4:1, indicating that the film is silicon-rich. The thermal stability of SiNx thin film was also investigated. The
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浙江大学硕士学位论文 王晓泉 2003年5月
experiments show that the film contains a large mount of hydrogen, but it will diffuse from the film after annealing at high temperature. Furthermore, the film is found possible to be cracked after annealing at high temperature(1000℃).
The experiments indicate that the deposition rate will increase with the increase of the flow ratio of SiH4/NH3, slightly decrease with the increase of substrate temperate, and increase obviously with the increase of RF power. The deposition rate is about 8.6 nm/min under some certain parameters. However, the refractive index will increase with the increase of the flow ratio of SiH4/NH3, slightly increase with the increase of substrate temperate, and decrease with the increase of RF power.
By measuring the passivation results of hydrogen plasma and SiNx thin film, we found an evident improvement of minor carrier lifetime in polycrystalline silicon after hydrogen plasma treatment, although it has little to do with the annealing temperature and time. The hydrogen contained in SiNx thin film can enhance the carrier mobility of monocrystalline silicon, but after annealing at high temperature the mobility turns down. SiNx thin film can improve the minor carrier lifetime of both mono and poly silicon by the simultaneous surface and bulk passivation. Both hydrogen plasma and SiNx thin film can effectively enhance the short circuit current density of mono and poly silicon solar cells, which cause the improvement of the absolute transfer efficiency about 0.5%~2.9%. Depositing SiNx thin film followed by hydrogen plasma treatment will result in better passivation effect.
The thickness of SiNx thin film will decrease and the refractive index will increase after annealing. But the refractive index will decrease dynamically at 1000℃. The post deposition of SiNx at 400℃ will enhance the hydrogen diffusion and passivation. Hydrogen will loss heavily and the minor carrier lifetime will decrease quickly at the temperature higher than 400℃. Hydrogen losses more obviously in the RTP (Rapid Thermal Process) than in the conventional furnace.
【Key Words】PECVD; silicon nitride; hydrogen passivation; solar cells
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